Optical fibers for data communication include a variety of fibers such as quartz glass, multi-component glass and plastic fibers. In practice, because of their light weight, low loss, durability, and high transmission capacity, quartz glass optical fibers are vastly used in a wide range of application. Since the quartz glass optical fibers, however, are very thin and susceptible to changes by external factors, quartz glass fibers as melt spun are generally provided with a primary coating and then with a secondary coating for protecting the primary coating. The primary coating is formed by applying a liquid curable resin of the type giving a soft cured product, followed by curing. The secondary coating is formed by applying a liquid curable resin of the type giving a hard cured product, followed by curing.
Properties required for the primary coating material include a low Young's modulus and low temperature dependency thereof for preventing microbending losses by external stresses or temperature changes, durability in terms of heat resistance and water resistance, low water absorption, low hydrogen generation, a high refractive index, and a fast-curing ability and low viscosity for allowing the drawing speed of optical fibers to be increased for improved productivity. To meet these requirements, UV-curable compositions based on urethane acrylate were proposed in the past. For example, JP-B 1-19694 and Japanese Patent Nos. 2,522,663 and 2,547,021 disclose liquid UV-curable compositions comprising a urethane acrylate oligomer, a reactive monomer, and a polymerization initiator. These compositions, however, fail to meet some of the above requirements, that is, a low Young's modulus and good low-temperature properties (minimized temperature dependency of Young's modulus), low water absorption, and low viscosity, because they are based on urethane acrylate oligomers of urethane bond-bearing polyethers or polyesters.
For reducing Young's modulus and improving low-temperature properties, JP-B 4-29619 corresponding to U.S. Pat. No. 4,496,210 and JP-A 61-21121 disclose liquid UV-curable compositions using a silicone urethane acrylate containing an organic polysiloxane. Urethane bonds are contained likewise. Because of the structural factors of urethane bonds (specifically, rigidity of the structure and the hydrogen bond in the urethane bond), these compositions are not satisfactory to some of the above requirements, that is, a low Young's modulus and a low viscosity. The embodiments described in these patents suggest that the organic polysiloxane is limited to a dimethylsiloxane skeleton, which has a low refractive index and is less compatible with reactive monomers. It is then difficult to design a liquid UV-curable composition capable of satisfying the required values.